1. Field of the Invention
This invention relates to a zoom lens suitable for a camera using a pickup tube or a solid-state image sensor, such as a video camera or an electronic still camera.
2. Description of Related Art
Rapid popularization in video cameras and electronic still cameras in recent years is largely attributable to their compact and lightweight designs. In order to increasingly popularize such cameras, it is necessary to facilitate further compact and lightweight designs and cost reductions of the cameras.
A zoom lens used in the latest video camera or electronic still camera requires conditions such as improvements in performance and magnification and an increase in diameter, not to speak of a compact and lightweight design and a cost reduction. In order to satisfy such requirements, it is necessary to realize a compact and lightweight design and a cost reduction of the entire lens system including a mechanical portion. For example, one of zoom lenses typical of the above case comprises a first lens unit with a positive refracting power, fixed when its magnification is changed; a second lens unit, called a variator, with a negative refracting power, moved when the magnification is changed and possessing a variable magnification function; a third lens unit, called a compensator, with a negative refracting power, moved when the magnification is changed and possessing the function of constantly holding the position of an image plane; and a fourth lens unit with a positive refracting power, fixed when the magnification is changed.
The specification of such a zoom lens is disclosed, for example, in each of Japanese Patent Preliminary Publications Nos. Hei 3-155516 and Hei 5-181062, and is such that an angle of view of incidence is about 50-60.degree. at an wide-angle position, an F-number is about 2, and a variable magnification ratio is about 5-11.
In addition to the above specification, the zoom lens used in the latest video camera or electronic still camera also requires a wider angle of view at the wide-angle position. For a zoom lens satisfying this requirement, there is a zoom lens for TV cameras, such as that set forth, for example, in Japanese Patent Preliminary Publication No. Hei 6-250086. This zoom lens includes four lens units with positive, negative, negative, and positive powers, arranged in this order from the object side, and is designed so that the angle of view of incidence is 80.degree. at the wide-angle position, the variable magnification ratio is 8, and the F-number is 1.7. In this zoom lens, however, the number of lenses used is large and thus the entire lens system becomes very bulky.
Japanese Patent Preliminary Publication No. Hei 8-234107 proposes a zoom lens which includes four lens units with positive, negative, negative, and positive powers, arranged in this order from the object side, and is designed so that the angle of view of incidence is about 60.degree. at the wide-angle position, the variable magnification ratio is 8, and the F-number is 2. This zoom lens is such that when the magnification is changed, a first lens unit is fixed, second to fourth lens units are moved, and a stop interposed between the second lens unit and the third lens unit is fixed with respect to an image plane. In this zoom lens, however, the number of lenses used is also large and thus the entire lens system becomes very bulky.
In general, in order to increase the angle of view of incidence at the wide-angle position and keep the diameter of the front lens of the first lens unit to a minimum at the same time in a zoom lens of such a type that four lens units with positive, negative, negative, and positive powers are arranged in this order from the object side, it is necessary to locate an entrance pupil as close to the first lens unit as possible at the wide-angle position. For this purpose, the negative refractive power of the second lens unit must be strengthened so that a ray of light imaged close to the maximum image height of the image plane is largely refracted at the wide-angle position by the second lens unit.
However, in a conventional zoom lens constructed with the four lens units with positive, negative, negative, and positive powers, as mentioned above, so that when the magnification is changed, the first and fourth lens units are fixed and the second and third lens units are move, the magnification is chiefly changed by the movement of the second lens unit, and thus the amount of movement of the second lens unit becomes considerable. Hence, if the negative refracting power of the second lens unit is strengthened, the light beam of the marginal angle of view, refracted by the second lens unit will be widely spread in the vicinity of the first lens unit with respect to the optical axis at a telephoto position. Consequently, when an attempt is made to prepare a wide-angle design in the conventional zoom lens constructed with the four lens units as described above, it is difficult to hold the balance of the position of the entrance pupil between the wide-angle position and the telephoto position. Furthermore, it becomes very difficult to correct aberration because the number of degrees of freedom of a lens system is highly limited when the magnification is changed. In order to solve the problem of correcting aberration, it is necessary to increase the number of lenses of each lens unit, but this causes increase in weight and cost. Moreover, the entire length of each lens unit must be reduced to hold a space for movement of each lens unit, and thus a vicious circle such that the front lens of the first lens unit must be enlarged may be caused.